This invention relates to conveyor ovens, and more particularly to devices for regulating the temperature and product transport speed in the conveyor type ovens typically used in food service and food product manufacturing applications.
Conveyor ovens are used in those applications which require that a relatively large volume of food be cooked at consistently controllable times and temperatures. A conveyor oven typically comprises a substantially enclosed heating chamber for cooking a food product at controlled temperatures, and a conveyor for transporting the food product through the heating chamber.
It may be desirable to cook the upper portion or surface of the product at a temperature different from that of the lower portion or surface. It may also be desirable to cook the product at different temperatures during portions of the cooking cycle. Accordingly, manufacturers of conveyor ovens now provide ovens having multiple heating zones. At least one having element is provided for each heating zone, and the elements within each zone may operate at a temperature which is selected independently of the other zones.
Such an oven typically has a control device which permits the user to select the temperatures for each zone and the speed at which the product is transported through the oven. Since the mechanical configuration of the heating elements remains constant, the product transport speed determines the length of time the product remains within each heating zone, and therefore controls the cooking time of the product.
The practical use of multiple-zone conveyor ovens presents a variety of significant control problems. In a commercial environment, it is especially important to be able to rapidly modify the oven's operating parameters so that different products may be prepared without a substantial setup time between products. For example, where a conveyor oven is used in a convenience store application to cook several types or sizes of food products for immediate service to a retail customer, it is essential that the oven's operating parameters may be changed to accommodate the next product immediately after the oven has finished cooking the previous product.
Existing conveyer ovens of which we are aware are equipped with control devices which require the user to individually adjust the operating set-points of the oven, such as the temperature of each zone and the product cooking time, whenever it is desired to change the parameters. For ovens having multiple zones, a relatively large number of set-point adjustments is required. As a result, an unreasonably large amount of time must be spent in adjusting the oven between products. This requirement is particularly troublesome in high-demand retail applications, such as convenience stores, where it reduces the throughput of the oven and also distracts the store clerk from other customer service duties.
In addition, because it may be desirable to cook a variety of products in the oven, it is necessary for the user to remember or to ascertain, the various oven operating parameters in order to adjust the set-points each time a different product is cooked. The set-points are typically expressed as numerical cook-time and temperature settings. Adjusting the set-points tends to be a difficult and error-prone process. The clerk may not understand the cooking instructions provided with the product, or may have difficulty translating that information into proper oven settings. Errors in the set-points typically result in improperly cooked food, and may even present safety problems, because food cooked at too high a temperature or for an excessive time may catch fire.
Because of these problems, some oven users choose to run the oven at a single consistent group or "recipe" of operating parameters all the time, regardless of the requirements of each food product cooked in the oven. For items which require more cooking than that provided in a single oven cycle, the user must cook the product through several cycles. This practice also typically results in improperly cooked food. In addition, this practice prevents optimal oven utilization, reducing oven throughput, increasing customer wait times, and increasing oven energy use.
Another problem associated with prior-art conveyor ovens relates to controlling the oven temperature developed by the heating elements. Conventional oven control systems included a user-adjustable thermostat device for regulating the temperature provided by the heating elements. The conventional thermostat measures the temperature in a selected region of the heating chamber and activates the heating elements whenever the measured temperature is below a user-adjusted predetermined temperature. Once the elements are activated, they produce heat and the temperature in the heating chamber begins to rise. After the measured temperature has risen a predefined amount above the activation temperature, the thermostat deactivates the elements. The predefined difference between the heating element activation and deactivation temperatures is referred to as hysteresis.
Because the heating elements are electrically operated, their activation and deactivation have been conventionally controlled using mechanical contacts which supplied electrical power to the elements. Mechanical contacts of this type are inherently slow, and subject to wear upon each operation. Accordingly, it has been generally preferred in the prior art to construct thermostats which provide substantial amounts of hysteresis between the heating element activation and deactivation temperatures, typically amounting to several degrees F., in order to reduce the frequency of operation of the contacts. Although this practice reduces contact wear and thus promotes reliability of the thermostat, it significantly increases the temperature excursions of the heating elements, which causes undesirable expansion and contraction of the heater elements themselves and also of other oven components. In addition, the substantial variations in heater element output and heating chamber temperature cause inconsistencies in cooking performance.
Yet another problem associated with prior-art ovens is accurate cook-time control. In order to accommodate the cooking of various food products, the cook-time must be user-selectable. In order to produce consistent cooking results for a particular product, the total cook-time must remain consistent each time that product is cooked. In multi-zone ovens, it is also necessary for each product to remain in each of the respective cooking zones for the same lengths of time on each cooking occasion. Prior-art conveyor ovens of which we are aware have generally not provided a user-adjustable cook time control which is sufficiently accurate and repeatable to satisfy these requirements.
In some prior-art ovens, cook time is controlled by a user-adjustable timer which simply enables the heating elements at the beginning of a cooking cycle, and disables the heating elements when the desired cook-time has elapsed. In such ovens, the cook-time is not directly related to the conveyor speed, but the conveyor speed need not be accurately controlled so long as the conveyor moves slowly enough that the product remains within the heating chamber for the entire cooking period. This method of controlling cook-time works poorly for multiple-zone ovens, however, because a single timer will not properly determine the length of time the product spends in each of the respective heating zones.
In other prior-art ovens, the heating elements are constantly enabled, and therefore the product is subject to cooking temperatures whenever it is within the heating chamber. Thus, the cook-time is controlled by varying the speed of the product transport conveyor. Such ovens may allow a user to select either a particular conveyor speed or a cook-time from which the controller determines the appropriate conveyor speed. A significant problem with such prior-art ovens is that it is difficult to accurately and repeatably control the conveyor speed over a wide variety of desired cook-times, power supply voltages, and conveyor loading factors.
The speed control problem is particularly great where long cook-times are desired, because long cook-times necessitate exceedingly slow conveyor speeds. The product transport conveyers in most conveyor ovens are driven by electric motors, and thus slow conveyor speeds require correspondingly slow motor speeds. Motor speed control methods used in prior-art conveyor ovens operate poorly at such low motor speeds, in part because at low speeds the effects of friction, conveyor loading, and other mechanical factors are maximized.
Another problem associated with prior-art conveyor speed controls is their inability to operate with a wide variety of oven configurations due to the differences in the motor types, conveyor drive gear ratios, and heating chamber lengths which are used in these ovens. Each of the prior-art conveyor speed control methods of which we are aware is designed for use with a particular combination of these parameters. Accordingly, substantial recalibration or modification of prior-art controls is required in order to make them compatible with a different motor, conveyor drive gear ratio, or heating chamber length. This increases maintenance costs, because a variety of control models must be stocked to accommodate various in oven configurations.
Another disadvantage of prior-art conveyor oven controls is inadequate fault detection and lack of tolerance for non-catastrophic fault conditions. When a fault occurs in an oven, it is essential that the fault be detected and the oven shut down if the fault presents a safety hazard. When the fault has no safety implications, however, it is desirable that the oven continue to operate if possible, even with some reduction in performance, until the oven can be repaired. Prior-art controllers generally have limited fault detection and handling capabilities. Some controllers do not even detect catastrophic faults, and therefore may be hazardous. Other controllers detect a wider variety of faults, but do not permit continued oven operation for non-safety-related faults.